Spinal muscular atrophy (SMA) is a genetic disease that causes muscle weakness and progressive loss of movement.

SMA is caused by deterioration in the nerve cells (motor neurones) connecting the brain and spinal cord to the body's muscles.

As the link between the nerves and muscles breaks down, the muscles used for activities such as crawling, walking, sitting up and moving the head become progressively weaker and shrink (atrophy). Mental abilities are unaffected by SMA.

The term spinal muscular atrophy can be applied to a number of different related conditions.

Classification of spinal muscular atrophy

SMA is classified according to the age symptoms appear and how much physical mobility a person has:

  • Type I – the most severe type. Symptoms appear in babies less than six months old, and they never develop the ability to sit unsupported.
  • Type II – less severe than type I. Symptoms usually appear in babies aged 7 to 18 months. Children with Type II can sit unsupported and some are able to stand, but they can't walk independently.
  • Type III – the mildest type affecting children. Symptoms usually appear after 18 months of age, and children are usually able to reach all the major motor milestones, including independent walking.
  • Type IV – affects adults.

In the most severe cases (Type I), severe respiratory problems mean children rarely survive beyond two years of age.

Type II SMA may shorten life expectancy, but improvements in care standards mean the majority of people can live long, fulfilling and productive lives. Survival into adulthood is now expected.

Life expectancy is usually unaffected in Types III and IV.

Read more about the symptoms of the different types of spinal muscular atrophy.

Management and support

It's not currently possible to treat the underlying genetic fault that causes SMA, although a number of experimental therapies are currently being evaluated in early clinical trials.

However, treatment and support is available to help manage the symptoms and provide people with SMA with the best possible quality of life.

Depending on its severity, treatment may involve:

  • exercises and equipment to improve mobility and breathing
  • feeding tubes and nutrition advice
  • bracing or surgery to treat curvature of the spine (scoliosis)

A range of healthcare professionals are involved in the care of a person with SMA, including:

  • doctors who specialise in neurology
  • physiotherapists
  • occupational therapists
  • speech and language therapists
  • if appropriate, respiratory physicians and orthopaedic surgeons

Read more about treating spinal muscular atrophy.

Testing for spinal muscular atrophy

If you have a family history of SMA, you should be able to access genetic testing. This involves checking a sample of your blood for the genetic fault responsible for the condition.

If there's a chance your baby could have SMA, it's also possible to check for this genetic fault during pregnancy, using tests such as chorionic villus sampling (CVS) and amniocentesis. Pre-implantation diagnosis (PGD) is also possible. Families should be referred to a geneticist to discuss all these options.

If a child shows typical signs of SMA, such as obvious muscle weakness, a blood test can usually confirm a diagnosis.

Read more about diagnosing spinal muscular atrophy.

Further information and support

The most common types of spinal muscular atrophy (SMA) affect an estimated 2,000 to 2,500 people in the UK. There are several charities for people with the condition, including:

Information about you

If you or your child has spinal muscular atrophy, your clinical team will pass information about you/your child on to the National Congenital Anomaly and Rare Diseases Registration Service (NCARDRS).

This helps scientists look for better ways to prevent and treat this condition. You can opt out of the register at any time.

Find out more about the register.

Page last reviewed: 09/03/2015

Next review due: 01/04/2017